BRITAIN'S top police officer warned MI6 it is not above the law as he revealed proposals for mass DNA screenings in the long-running Gareth Williams investigation.

An independent forensics review will form a central part of fresh efforts to solve the 21-month inquiry into how the codebreaker's body ended up in a holdall, Scotland Yard's Commissioner said.

Bernard Hogan-Howe has also told detectives to deal directly with the intelligence agency in a break with tradition at the Metropolitan Police.

Homicide detectives were previously forced to involve counter-terror colleagues in a bid to obtain statements and evidence from MI6.

But Mr Hogan-Howe was angered by the "unacceptable" breakdown in communication which saw evidence fail to come to the senior investigating officer until last week at an inquest.

When asked what powers he had to ensure MI6 co-operated, he told reporters: "It's the law." He said mass screening in the case would be carried out on a "voluntary" basis.

Mr Hogan-Howe said: "Of course it may well be that Gareth Williams' death has nothing to do with employment. All we need to do is to make sure that all areas of his life were fully explored."

Mr Hogan-Howe said forensics firm LGC, which was responsible for a mix-up early in the investigation, would not be in charge of the review.

But he added: "This is not about criticising the forensic system."

Detective Chief Inspector Jackie Sebire, who has led the investigation since the body was found in August 2010, is likely to pass on the case to a colleague because she is being promoted.

Members of the secret services have came under fresh scrutiny after the coroner at last week's inquest said she was sure a third party locked Mr Williams inside the red holdall in which his naked body was found in his bathtub.

Giving her verdict, Dr Fiona Wilcox said the 31-year-old was probably killed and it "remained a legitimate line of inquiry" that the secret services may have been involved in the death.

But inquiries have yet to yield a culprit, with forensic experts still hoping for a breakthrough from DNA tests on a green towel discovered in his kitchen.

Mr Williams, a fitness enthusiast originally from Anglesey, North Wales, was found in the bag in his flat in Pimlico, central London.